1. Field of the Invention
The present invention relates to an automatic focus adjusting device for use in a camera, and more particularly to an automatic focusing camera which permits flash photography by means of an electronic flash device.
2. Description of the Prior Art
Known in the prior art are various types of range-finding devices which measure a camera-to-target object distance on receipt of light from the target object with a light receiving device, and which generate a distance signal in response there-to. Also known are various types of automatic focusing cameras which automatically focus an objective lens according to the distance signal. In many cases, however, the distance measuring in such range-finding devices is performed by use of a target object image formed on the light receiving device. Therefore, when the contrast of the target object itself is lower than a predetermined level, the range-finding devices do not generate any distance signal, resulting in no focus adjusting suitable for the target object.
U.S. Pat. No. 4,183,643 discloses an automatic focus adjusting device which obviates such inconvenience. This device is of a type that premeasures a camera-to-target object distance with a range-finding device, and thereafter, shifts an objective lens from the infinity focusing position toward the closest focusing position, and then stops the objective lens at a position corresponding to the distance signal generated by the range-finding device. Furthermore, such automatic focus adjusting device is constructed such that the objective lens is automatically set to a pan-focal position when the range-finding device generates no distance signal. According to the device, the objective lens is set to a pan-focal position even when no distance signal is generated by the range-finding device due to the low contrast of the target object. However, in this device, the distance measuring and object lens position adjustment are carried out in turn, thus requiring a longer time for focus adjustment. There have also been various types of arrangements known for shifting a movable lens together with the scanning of a camera-to-target object distance for its measurement. The following problems, however, occur in an arrangement in which a range-finding device scans the distance from a far position toward a closer point, while the movable lens is shifted from the infinity focusing position toward the closest focusing position in response to the scanning of the range-finding device:
(1) When the contrast of a target object image formed on the light receiving device is lower than a predetermined level, no distance signal is generated. As a result, the movable lens included in the objective lens is shifted to its closest focusing position, causing the objective lens to be set in the closest focusing condition. However, generally, the farther a target object is located, the lower the contrast of the image formed on the light receiving device becomes. Therefore, it is often true that the farther the target object is, the more the distance signal is unobtainable. This causes the objective lens often to be set in the closest focusing condition while the target object is positioned afar.
(2) In flash photography using an electronic flash device, when the range-finding device fails to measure a camera-to-target object distance and generates a distance signal relative to the background of the target object, incorrect exposure results if the distance to the background is larger than the maximum permissible distance for flash photography.
It is to be noted that such problems occur not only in the distance-measuring type of automatic focus adjusting devices, but also in the focus detecting type that includes a focusing condition detecting device for detecting the focusing condition of the objective lens based on the contrast of an image formed by the objective lens, while shifting the movable lens from the infinity focusing position toward the closest focusing position and stopping it when an in-focus signal is generated from the focusing condition detecting device.